In multiple input multiple output (MIMO) wireless systems, multiple data streams may be transmitted simultaneously using a plurality of transmitting antennas. A MIMO receiver may utilize a plurality of receiving antennas to decouple, and detect individual data streams. Two predominant methods for MIMO transmission include singular value decomposition (SVD), and layers space-time (LST) processing, also known as successive interference cancellation (SIC).
SVD may use beamforming in conjunction with a transmitter antenna array and receiver antenna array to create virtual channels, or eigen-channels, through which multiple data streams may be sent without interfering with one another. LST/SIC may use receiver antennal array processing to detect the multiple data streams, one stream at a time. For each detection “layer,” the interference from yet undetected streams may be nulled out, while the interference from already detected streams may be cancelled, or subtracted, out.
The eigen-channels in SVD may have highly unequal signal to noise ratios (SNR), depending on the eigen-spread of the MIMO channel matrix. SVD may also rely upon adaptive modulation, or adaptive bit loading, to achieve greater data transfer rates for eigen-channels associated with higher SNR values, while simultaneously supporting lower data transfer rates for eigen-channels associated with lower SNR values. SVD may also suffer performance loss, by not achieving the peak theoretical data transfer rate aggregated among the eigen-channels when a broad range of modulation types are not available. For example, if a maximum data transfer rate associated with an eigen-channel requires a 1024 QAM modulation type, the maximum data transfer rate may not be achievable if the required modulation type is not available to be utilized.
LST/SIC approaches may suffer performance losses as a result of error propagation. For example, if a current layer is detected in error, the error may propagate to other layers increasing the probability that subsequent layers may also be detected in error. LST/SIC may require stream re-ordering to detect data streams with higher SNR values first to minimize error propagation. Some methods, such as vertical LST (VLST) may provide error protection through coding of each data stream. Based on the coding, decisions may be made subsequent to decoding to subtract out interference.
Alternatively, precoding based on dirty paper theory, for example Tomlinson-Harashima precoding (THP), may be utilized to pre-cancel interference at the transmitter without requiring the signals to be transmitted with greater levels of transmitted radiated power. The THP approach may require channel knowledge at the transmitter.
Geometric mean decomposition (GMD) may utilize beamforming and LST/SIC at transmitter, by utilizing THP for example, or at a receiver, by utilizing VLST for example. SNRs for each of a transmitted plurality of data streams may be about equal when utilizing GMD. Consequently, adaptive bit loading may not be required as may be the case with SVD. GMD may also not require reordering of data streams as may be the case with LST/SIC. GMD may achieve data transfer rates that are approximately equal to the channel capacity.
Circuitry within a MIMO transmitter may cause noise to be incorporated in transmitted signals. The noise may be referred to as “transmitter-induced noise.” A potential cause of transmitter-induced noise may include nonlinearity in the output signal gain of power amplifiers as a function of input signals. Another potential cause of transmitter-induced noise may be phase errors between corresponding in-phase (I) and quadrature phase (Q) signals generated by the MIMO transmitter. These phase errors may be referred to as “phase noise.” In general, an error in a magnitude and/or phase for an I signal, and/or an error in a magnitude and/or phase for a corresponding Q signal may be referred to as an “IQ imbalance”. IQ imbalance is also a potential a cause of transmitter-induced noise. A measure of transmitter-induced noise is an error vector magnitude (EVM) as defined in IEEE resolution 802.11n, for example.
Some conventional MIMO receivers may attempt to decode information contained in received signals without compensating for transmitter-induced noise. A result may be errors that occur during the decoding of the information. Measures of errors at a MIMO receiver during decoding may include bit error rate (BER) and packet error rate (PER).
Techniques utilized in some conventional MIMO receivers based on LST/SIC may require stringent limitations on transmitter EVM to allow the MIMO receiver to achieve acceptable BER and/or PER rates when receiving and/or decoding received signals. Meeting these stringent limitations may require the utilization of expensive circuitry in the MIMO transmitter.
Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of such systems with some aspects of the present invention as set forth in the remainder of the present application with reference to the drawings.